Inductive systems such as metal detectors typically use magnetic effects such as the variation of mutual inductance in a multi coil system or the induced current on a single coil system resulted due to a presence of a metallic target.
The signal measured by a metal detector, for example, corresponds to an alteration in a magnetic field due to the presence of the metallic object to be detected, therefore, introducing additional components and/or systems with metallic content and or/circuitry has several adverse effects. One such effect is the lowering of the metal detection usable dynamic range—for example if one introduces a metallic element in the detection coil's vicinity it will interact with the coil's magnetic field. This interaction may generate signals which are comparable in magnitude to the signals due to objects we wish to detect, thus limiting the maximum detector gain and, correspondingly, the weakest signal and target detectable.
When metallic element is an electronic circuit, parasitic signals may be induced to the coils by the currents in the electronic circuits, especially when switched signals result in magnetic field transients. For example, DC/DC converter circuits may use switched inductors in order to regulate the voltage. A typical architecture may involve deliberately introducing a discontinuity in voltage on an inductor or transformer, with a corresponding switching of the inductor's magnetic field.
Such variations in magnetic field are then picked up as parasitic signals masking the wanted signals.
Moreover, the dynamic nature of the signals—such as its increase and decrease—is often used in detection algorithms. The addition of dynamic parasitic signals may hinder the operation of such algorithms.
Another adverse effect associated with induced currents may be the damaging of components in the electronic circuits. Metal detectors often generate strong switched magnetic fields using their coils, which may induce high voltage levels within the electronic circuits. The high voltages may harm sensitive components within the electronic circuits. Also, signals induced by the metal detector coils may interfere with desired signals within the circuits and harm circuits' performance.
The prior art solutions that are currently used to overcome such problems, rely on magnetic shielding of the electronic circuits and/or system. An example of the magnetic shielding solution is illustrated in US patent application publication number 2013/0057286 to Alexander Lewis Jones et al. entitled “Metal detector sensor head”. The disadvantage of applying the shielding disclosed by this publication is that it requires special and expensive materials. Moreover, magnetic shielding increases the weight of the product. Additionally, the shielding provides only a partial solution due to a decline of loss over large bandwidths.